A. Field of the Invention
The present invention relates to a sheet-fed rotary press for one side/perfecting printing, and particularly to a reversing gripper appratus provided on a reversing cylinder which composes a paper transfer means.
B. Description of the Prior Art
A sheet-fed rotary press for one side and perfecting printing is adapted for multicolor printing of two or more colors on one side of a sheet of paper or for printing of one or more colors on each of two sides of a sheet of paper. Such presses are capable of wide application and are thus winning high public favor.
This one side/perfecting printing press comprises a plurality of printing units which are connected by a paper transfer means comprising paper transfer cylinders, double-diameter cylinders, and reversing cylinders. These papar transfer means cooperatively effect the paper transfer and paper reversing which is required for perfecting printing.
Namely, in the case of one-side printing, a gripper at the front end of the double-diameter cylinder grips the front edge of a sheet and makes the reversing gripper of the reversing cylinder grip the front edge at a point of contact with the reversing cylinder. The reversing gripper then rotates together with the reversing cylinder while gripping the front edge of the sheet and delivers it to the impression cylinder of a following printing unit. However, in the case of perfecting printing, the front-end gripper of the double-diameter cylinder passes through the above-described contact point while gripping the sheet, and then delivers the rear edge of the sheet to the reversing gripper of the reversing cylinder when the rear-end gripper reaches the above-described contact point, whereupon the reversing gripper delivers the sheet to the impression cylinder of a following printing unit after the sheet is reversed as the reversing cylinder rotates.
Therefore, such reversing gripper apparatus provided on the reversing cylinder differs from those provided on the double-diameter and paper transfer cylinders which only carry out paper receiving and delivery and thus requires suitable opening-closing operations (gripping and releasing) corresponding to to both the case of one-side printing and the case of perfecting printing, as well as a reversing function for paper in perfecting printing, so that there is a need for consideration of an arrangement which entirely differs from that of other grippers.
With regard to the reversing gripper, U.S. Patent publication No. 3,537,391 discloses an art wherein two pairs of rotatable tumbler grippers are adjacently provided in a reversing cylinder. In this prior art, only one of the two pairs of tumbler grippers is driven in one-side printing and the other of the pairs is completely stopped. However, in perfecting printing, one of the pairs of tumbler grippers receives the rear edge of a sheet, the other tumbler gripper being caused to grip the rear edge of the sheet to effect delivery, whereby the sheet is delivered to the impression cylinder of a following printing unit after the sheet is reversed as the reversing cylinder rotates.
However, this prior art involves a situation where the function of gripping paper between the two pairs of tumbler grippers provided in the revering cylinder should be changed in one-side printing. Thus there is an increase in the number of times that sheet is delivered, and this consequently produces disadvantages in terms of delivery error and reduction in register accuracy.
In addition, the tumbler grippers which rotate as a combination of the grippers and the gripper pads have poor reliability in terms of the gripping function because the gripper pads are not fixed.
Furthermore, since the two pairs of tumbler grippers rotate in the circumferential direction of the reversing cylinder, errors produced in the stop positions of these tumbler grippers will translate as a register error with respect to the sheet and will subtly affect the quality of printing.
Other representative prior arts relating to the reversing gripper apparatus, West Germany patent application No. P2414998.2 and U.S. Pat. No. 3,899,970, disclose reversing grippers which comprise two combined grippers rotatable around a common shaft. One of these combined grippers operates as a gripper and the other as a gripper pad, the combined grippers rotating around a common axis and reversing the sheet in perfecting printing.
However, in an analysis of the prior arts from the viewpoint of gripping function, it is found that since the sheet is separately swung by an opening and closing mechanism at each time of gripping and releasing, the position relative to the gripper pads of the cylinder which is the object of receiving and delivery is easily moved, so that there is some difficulty in obtaining an accurate receiving and delivery operation and instability with respect to the register accuracy.
In an analysis of the gripping force, since the prior arts are so constructed that a spring is interposed between the combined grippers to produce the gripping force and the combined grippers are operated by an opening-closing control mechanism comprising a member such as a segment gear which acts against the force of the spring, it is impossible to make the spring any stronger than a definite limit which is defined by the opening-closing control mechanism or ultimately to set the gripping force at a sufficiently large value.
In addition, sheets of various thicknesses are used in a printing press, and thus, particularly when the sheet employed is thick, any discrepancy in the position of the gripper point is inevitably increased between the combined grippers and the grippers of the opposite cylinder in the radial direction thereof. Consequently, a wavelike difference in height is produced at the gripping position (namely, the edge) of the sheet, resulting in the occurrence of such printing problems as doubling and poor register.
During the reversing operation, the combined grippers cooperatively deliver the sheet to the grippers of the opposite cylinder, and thus it is difficult to ensure the positional accuracy of the combined grippers, do that there is a need for a fine regulating operation.